Target System Temps

[Tennman]

Last season was a flurry of events feeding the beast, fixin' issues, learning about the Danfoss, on and on. This season I have digital sensors at the boiler exit and return in the barn and a digital sensor on the supply into and out of the HX. I'm about to install a digital thermocouple just above the chimney tee coming out of the back of the boiler. So... now that I have the tools to see how everything is running what are "good" target temps for EVERYTHING. I've read discussions about return water temps, exhaust stack, boiler exits and large/small deltaTs across the HX. So for me and others here in one place what should I see for a good working system.

Exhaust Gas Temp just as it turns to go vertical in the chimney = ?
Exit water temp at the boiler (last year I set at 185F because underground sucked) = ?
HX deltaT (HXinlet - HXoutlet) = ?

Fingers crossed for good Boiler barn to house and return deltaTs. Will report on performance of my foam sticky job with deltaT loss/ft. Not looking forward to cold weather... but sorta itching to see how my system improvements and my experience helps the system here in my second season.

Great time of year.... football... deer season... Thanksgiving/Xmas... shed full of wood, and boiler runnin'! Cheers from the South East Conference!

 

[Nofossil]

Sheesh - nobody taking this one? I'm no expert on this, but here's my $0.02:

Short answer is that there is no short answer.

Flue temps - want to be as low as you can get without condensing in the chimney (not an issue for most systems). Each boiler is different, and different firing rates, draft, and fuel will affect the actual temp, so you'll have to learn what's normal for yours. Given a baseline, higher temps probably mean lost efficiency - fly ash in the fire tubes, less than ideal primary / secondary air, and so on.

Exit temp - if you can control it, lower means better efficiency as long as it's high enough to get the job done.

HX delta T - as large as possible. Again, you'll need to establish a baseline. All other things being equal, a lower delta T probably means lost transfer efficiency (maybe time to clean the HX). You might want to slow down the circulator to get a larger delta T (will be larger both at the boiler and at the HX). At some point you can't move enough heat and/or your boiler outlet temp starts getting too hot (see above).

 

[jebatty]

I'll second Mod's reply with these comments.

Exhaust Gas Temp: IMO the goal is to get good, continuous gasification with the heat output you need. In my case the short answer is stack temp 360-425F, measured about 1 foot vertical from the L exiting the boiler, sensor in the middle of the stack. I have the home-made chain turbulators, so stack temps may be up to 100F less than if no turbulators. Stack temp will be affected by chimney draft, moisture content of wood, how full the firebox is, size of splits, stage of burn, primary/secondary air settings, water flow rate through the boiler, and more. My boiler hot water output will be higher with higher stack temps, but overall efficiency may be somewhat lower. From some source I recall that in the normal operating range an increase in flue temp is inversely proportional to efficiency, i.e., a 100F increase in flue temp = 3% reduction in efficiency, and vice versa. But in a particular situation the higher flue temp may be needed to get needed hotter water output. Everything else being equal, my hot water output temp will be higher with higher flue temp.

Exit Water Temp: Similar to above, IMO the goal is to get water at the temp needed, and no higher, to meet heat demand, IF no storage. If storage, then the goal will be to achieve storage temp/btu's to meet heat demand during periods then the boiler is not firing, and therefore output needs to be at a higher temp than demand to achieve stored btu's. With return water protection (150-160F typical) my boiler output never is lower than 160F, and typically will be 165-170F with system/storage return below 130F. As return rises, boiler output rises, up to a maximum of around 190F. Other things being equal boiler output will increase with increase in rate of burn and and decrease as water rate of flow increases, and vice versa.

In my system, so long as return from storage is below 170F, my Tarm can operate at full output without idling, delivering all output to storage, and result in 190F water from top of tank down. As return from storage rises above 170F, at high burn I will experience idle periods, but total tank temp will rise. I am able, with wood load management, to heat my entire tank up to 190F +/-, top to bottom. This is not a normal practice, however. I normally charge the tank to the point of storage return of about 170-175F.

HX deltaT (HXinlet - HXoutlet): IMO the goal of sizing a plate HX (probably any HX) is to achieve needed btu output to system. This will relate to deltaT and required flow rates on both Sides A and B of the HX. The normal standard for sizing a plate HX appears to be a size which results in a 10F approach temp (Side A inlet minus Side B outlet = 10F), or if 170F water inlet to Side A from boiler, Side B outlet to system will be 160F. This depends on proper sizing of the HX to flow rates on Sides A and B to achieve needed Side B btu output to system. Other than cost, I don't think there is a disadvantage to over-sizing an HX, but under-sizing results in a real problem, or impossibility, in delivering needed system btu's.

 

[Tennman]

thanks for the info guys. I've been swamped the last few days. I've not drilled my chimney yet for the stack thermocouple so the 1' above the tee is helpful. Last season I had my contoller set at 185 since I had such large transit losses. May go back to 180 exit. appreciate the replies. proabably several weeks for firing, but getting cool here.

 

[mwk1000]

I have a magnet thermometer on the stack that I never see above 200, starting to think it's a piece of junk, it was cheap. As far as the rest of them they cycle over the life of the burn for me since it's all about heating a pretty big chunk of water.

At 140 the boiler pump comes on , after the lines have cycled and we are back to 140 the work starts. The danfoss valve chokes the flow to the HX when starting the HX will have 20-30 degree dletas on the bouler side with 10 degrees on the tank side.

After the beast is heated up incoming water stays a 165/170 for quite a while as the HX return to boiler is quite low 105. The Return at the boiler stays at 140 as the mixing valve does its job. The HX boiler side will have up to 50-60 degree delta while the flow is restricted.

As the tank is heated and the incoming water temp rises ( to the boiler ) I will see 40 degree delta on the TANK side is when the tank bottom is at 120-140 , once the return is above 140 to the boiler the transfer to the tank is a delta of about 30 on the tank and 10-25 on the boiler ( It has a larger pump/higher flow ).

I Can reload eko about then to push it to 180 at the tank top but the transfer rates will consistently drop and my feeling is that I am getting nowhere near the BTU's ( So say my graphs of the delta T at 10 degrees or less ) . So I only do it on very cold days when I need the extra hours.

So all in all I use the tank side HX delta T to decide when I am getting the most. Since that is the real saved heat. The peak I see is 40ish that makes me happy to see so much heat getting stored. ( I have my logger do the subtraction so I see the Delta values every 1 min. right next to the temps )

But the moral of my story is that I have no fixed temps to focus on as much as understanding the cycle, the whole thing is a slow rise and fall. When the boiler is running full blast and is hot and only shows 160 I know that the cold tank water is pulling away a huge amount of heat! When the boiler is at 190 I know I am done getting the most.

I made a chart of the Source HX Delta T and the Load HX delta T . It looks like a skewed sine wave and The process is over when the lines get to within 5 degrees of each other.

Here is yesterdays peak at 38.7
Requesting temperatures... DONE
SourceFeed higher than LoadFeed - set relay flag to HIGH/ON 167/111
log:SourceFeed:F:167.00
log:SourceReturn:F:121.10:45.90
log:LoadFeed:F:106.70
log:LoadReturn:F:145.40:38.70
log:BoilerFeed:F:168.80
log:BoilerReturn:F:157.10